First, register an account here. This creates a profile for you on their website. The profile links your BlackBerry device to you. After this you need to activate your account. Check the email you received when you signed up.

Second, install the tracker on your BlackBerry. You can download the app OTA by going to http://blackberrytracker.com/download in your BlackBerry browser. Pick the OS that is right for your device (either OS 4.1 or 4.2). If you're not sure what OS you are running, go to the options menu under settings on your BlackBerry. In options to go the about option and you should see your OS specified for you.

Next you need to configure the tracker client. Click the BlackBerry Tracker icon on your home screen. Then login using the username that you created in step 1. Once you're logged in, the app is all setup and ready to go!

Some key features noted on their site:

Live Tracking
The core feature of Blackberry Tracker is live tracking of your Blackberry device. Once the client application is running on your phone, GPS coordinates will then be sent to the server at the interval you specify on your phone(Ex. 30 seconds, 60 seconds, etc,.).

Tracking History
After the application has been running on your phone, you will begin to accumulate a history of GPS data. You can see the path of your phone between any two dates. On the home page select Tracking History.

Track Friends
Want to let your friends see your location? Now you can with Blackberry Tracker. Start off by having your friends sign up for an account. After they have signed up, all they need to do is search for you under Find User and then select Track this user's location. After they have permission from you, they can go under Track Friends on the home page to begin finding your location!